For a 12V setup a 20Ah battery (either a single one or several smaller parallelled), you basically wouldn't need a charger if you have a 10-15W amorphous solar panel to keep them topped off.

At any rate, 20Ah should give you about 60-80 hours total running at maximum music output at 12V. So at the most you're looking at having to recharge the batteries maybe once a week if you remember to turn it off in between listening sessions.

Any battery charger that can charge 12V batteries would do for that, just disconnect it after ended charging.